数据结构_线索化二叉树

• n个结点的二叉链表中含有2n-(n-1)=n+1 个空指针域。利用二叉链表中的空指针域，存放指向该结点在某种遍历次序下的前驱和后继结点的指针（这种附加的指针称为"线索"）
• 这种加上了线索的二叉链表称为线索链表，相应的二叉树称为线索二叉树(Threaded BinaryTree)。根据线索性质的不同，线索二叉树可分为前序线索二叉树、中序线索二叉树和后序线索二叉树三种。

public class ThreadedBinaryTree {
	private TreeNode root;
	private TreeNode pre;

	public ThreadedBinaryTree(TreeNode root) {
		super();
		this.root = root;
	}

	/**
	 * 中序线索化遍历
	 */
	public void infixList() {
		TreeNode cur = root;
		while (cur != null) {
			while (cur.leftType == 0) {
				cur = cur.left;
			}
			System.out.println(cur);
			while (cur.rightType == 1) {
				cur = cur.right;
				System.out.println(cur);
			}
			cur = cur.right;
		}
	}

	/**
	 * 中序线索化
	 * @param treeNode
	 */
	public void infixThreadedTreeNodes(TreeNode treeNode) {
		if (treeNode == null) {
			return;
		}
		//线索化左子树
		infixThreadedTreeNodes(treeNode.left);
		//处理当前节点前驱节点
		if (treeNode.left == null) {
			treeNode.left = pre;
			treeNode.leftType = treeNode.PRECURSOR_OR_SUCCESSOR;
		}
		//处理当前节点后继节点，实际是在递归的上一层处理的
		if (pre != null && pre.right == null) {
			pre.right = treeNode;
			pre.rightType = treeNode.PRECURSOR_OR_SUCCESSOR;
		}
		//使当前节点称为下一个节点的先驱节点
		pre = treeNode;
		//线索化右子树
		infixThreadedTreeNodes(treeNode.right);
	}

}

class TreeNode {
	int val;
	TreeNode left;
	TreeNode right;
	//左右指向的类型
	int leftType;
	int rightType;
	//0表示子树，1表示前驱或者后继节点
	static final int SUBTREE = 0;
	static final int PRECURSOR_OR_SUCCESSOR = 1;

	public TreeNode(int val) {
		super();
		this.val = val;
	}

	@Override
	public String toString() {
		return "TreeNode [val=" + val + "]";
	}
}